In a boat that moves by the use of an outboard engine, when the boat is at a standstill and when the boat begins to move, the level of the stern of the boat drops and sinks into the water, and the bow rises and is tilted upward. Since the hull therefore begins to move in a tilted state, the water resistance is considerable when traveling starts and adequate boat speed cannot be obtained. The stern must rise upward a certain amount and the orientation of the boat must become approximately horizontal in order to reach a certain level of speed. There is a problem in that time is required for the boat to approximate an orientation that is nearly horizontal, and the boat cannot smoothly accelerate.
An outboard engine that can improve the acceleration characteristics of a boat is disclosed in Japanese Patent Laid-Open Publication No. 5-319386 (JP-5-319386A) and Japanese Utility Model Laid-Open Publication No. 47-9194 (JP-UM-47-9194A).
In the outboard engine of the 5-319386A publication, an engine, a vertically disposed drive shaft and other drive components, and transmission components are covered by a vertical cowling. A propulsion casing is disposed below the lower cowling so as to provide vertical linkage. When the boat is at a standstill, a portion of the lower cowling is submerged, and when the boat is moving, only the propulsion casing is submerged.
In the outboard engine of the 47-9194A publication, the waterproof engine casing that covers the engine is formed having a size that is sufficient to provide flotation to the engine, and the engine is designed to float on the surface of the water.
In the outboard engine of the 5-319386A publication, however, a portion of the lower cowling that forms the engine room is structured to submerge and it is therefore difficult to endow this structure with water tightness when the lower cowling is assembled. When water has furthermore flooded the engine room, it is difficult to drain the water, the movement of movable components is compromised by water and salt, and the components tend to corrode.
Thus, when a buoyant member that has volume is provided to the outboard engine main body in an outboard engine mounted on the stern, the buoyant member is mounted on a bottom case positioned below the engine room, and an extension case positioned below the bottom case. The width of the outboard engine is accordingly increased from the middle portion in the vertical direction of the outboard engine to the bottom portion of the engine. When such wide outboard engines are mounted in parallel on the stern, there is a danger of the adjacent buoyant members of the outboard engines creating interference when any of the outboard engines is tilted up or turned for steering in order to perform maintenance or storage.
In view of the above, it is necessary to provide an engine in which the extension casing and other watertight structures are not affected, the extent to which the stern of a boat is submerged during stopping or acceleration can be reduced, and the hull orientation can be rapidly brought to a near-horizontal state during acceleration; and in which the buoyant member of an adjacent outboard engine is prevented from creating interference during tilting or the like when a plurality of outboard engines is mounted in parallel on the stern.